Depletion of human peripheral blood leukocytes (PBL) of Leu M3+ cells by adherence to plastic and sephadex G10 results in the abrogation of proliferative (3H) and cytotoxic T lymphocyte (CTL) responses to influenza A virus (FLU), but in an elevation of 3H and CTL responses to HLA alloantigens (ALLO). This loss of the elevation of 3H and CTL responses to HLA alloantigens (ALLO). This loss of the FLU T cell response was attributed to removal of antigen-presenting cells (APC), whereas the elevation of the ALLO response was shown to be due to suppressor cell (or a suppressor inducer cell) that is contained in the Leu M3+ adherent cell population. Suppressor activity was inactivated by culturing either unfractionated PBL or adherent cells with viruses, including influenza A, measles and mumps viruses.